In the last year, roughly two billion digital imaging systems were sold worldwide, and over a trillion images are now on the Internet. In addition to photography, digital imaging is transforming numerous fields, including entertainment, social networking, ecommerce, security and autonomous navigation. In the coming years, ubiquitous digital imaging systems may transform diverse fields such as personalized medicine, wearable devices, smart environments, situational awareness, sensor networks and scientific imaging.
Conventional image sensors often use pixels that include photodiodes operating in photoconductive mode to generate image data. For example, FIG. 1 shows an example of a conventional three transistor (3T) pixel. As shown in this figure, conventional pixel 100 can include a photodiode 102 with the anode connected to ground and the cathode connected to a source of a reset transistor 104 and a gate of a source follower transistor 106. A drain of reset transistor 104 is connected to a voltage Vdd and a gate of reset transistor 104 is connected to a reset line to which a reset signal Vres can be applied. A drain of source follower transistor 106 is connected the Vdd and a source of source follower transistor 106 is connected to a drain of a read-out transistor 108. A gate of read-out transistor 108 can be connected to a row selection line to which a selection signal Vsel can be applied, and a source of read-out transistor 108 can be connected to a column bus.
Before capturing image data with conventional pixel 100, a reset signal is applied to the gate of reset transistor 104, which causes voltage Vdd to be applied to the cathode of photodiode 102. When the reset signal is removed, the voltage at the cathode of photodiode 102 is equal to Vdd, which reverse-biases photodiode 102. When light is incident on photodiode 102, a current is induced from the cathode to the anode and the voltage across photodiode 102 drops from Vdd by an amount that is proportional to the incident light energy and exposure time. The voltage at the cathode is buffered by source follower transistor 106 and is read out to the column bus as Vout when a signal Vsel is applied to a read-out transistor 108.
An illustration of currents present in a photodiode is presented in FIG. 2. As shown, a photodiode is typically a P-N junction semiconductor which can be modeled as an ordinary diode D, with a capacitance C, shunt resistance Rsh, and series resistance Rse. When the photodiode is connected to an external load, current flows from the anode through the load and back to the cathode. The total current that flows through the photodiode is the sum of the photocurrent Ipd (due to light) and the dark current Id (due to a bias voltage applied across the photodiode).
Conventional pixel 100 consumes power during reset and readout due to the application of Vdd, Vres and Vsel. Additionally, due to the reverse bias of photodiode 102, a “dark” current is generated even when light is not incident on photodiode 102, which can cause noise in an image generated from Vout. Therefore, in order to operate an image sensor using conventional pixels 100, an external supply of power is required as the pixels each consume power during operation. Such an external power supply is typically either a power supply connected to an electrical grid, or a battery that is charged from the electrical grid. As such, image sensors using conventional pixels are not suitable for applications in which a power supply is inaccessible and/or applications in which it would be difficult to recharge a battery. Moreover, even in applications where a battery can be recharged and/or a power supply is accessible, use of image sensors using conventional pixels can be an undue drain on the available power supply, but are necessary due to a lack of a useful self-powered image sensors.
Accordingly, it is desirable to provide new circuits for self-powered image sensors.